Techniques for reducing read voltage threshold calibration in non-volatile memory

ABSTRACT

A non-volatile memory includes a plurality of cells each individually capable of storing multiple bits of data including bits of multiple physical pages including at least a first page and a second page. A controller of the non-volatile memory determines a first calibration interval for a first read voltage threshold defining a bit value in the first page and a different second calibration interval for a second read voltage threshold defining a bit value in the second page. The second calibration interval has a shorter duration than the first calibration interval. The controller calibrates the first and second read voltage thresholds for the plurality of memory cells in the non-volatile memory based on the determined first and second calibration intervals.

BACKGROUND OF THE INVENTION

This disclosure relates to data processing and data storage, and more specifically, to calibration of one or more read voltage thresholds for a unit of data storage in a non-volatile memory system. Still more particularly, the disclosure relates to techniques for reducing the overhead of a read voltage threshold calibration in a non-volatile memory system.

NAND flash memory is an electrically programmable and erasable non-volatile memory technology that stores one or more bits of data per memory cell as a charge on the floating gate of a transistor or a similar charge trap structure. The amount of charge on the floating gate modulates the threshold voltage of the transistor. By applying a proper read voltage and measuring the amount of current, the programmed threshold voltage of the memory cell can be determined and thus the stored information can be detected. Memories storing one, two, three and four bits per cell are respectively referred to in the art as Single Level Cell (SLC), Multi-Level Cell (MLC), Three Level Cell (TLC), and Quad Level Cell (QLC) memories. In a typical implementation, a NAND flash memory array is organized in blocks (also referred to as “erase blocks”) of physical memory, each of which includes multiple physical pages each in turn containing a multiplicity of memory cells. By virtue of the arrangement of the word and bit lines utilized to access memory cells, flash memory arrays have generally been programmed on a page basis, but erased on a block basis.

In multi-level (i.e., MLC, TLC and QLC) NAND flash memory, information is stored by programming the memory cells to various quantized threshold voltage levels according to the device's programming algorithm, which maps the binary bit values to discrete threshold voltage levels. In response to a page read command, the binary bit values are retrieved by applying appropriate read voltages that divide the programmed threshold voltage window into discrete regimes and by then applying a reverse mapping between the detected threshold voltage levels and the corresponding binary bit values. Over the lifetime of a multi-level NAND flash memory device, the distributions of programmed threshold voltage generally become degraded due to the effects of wear on the memory cells. Consequently, it is generally desirable to adapt or calibrate the read voltage thresholds defining the various bit values over time to compensate for the effects of wear and to extend the useful life of the NAND memory device.

BRIEF SUMMARY

In conventional NAND flash memory devices, each page is calibrated on the same iterative schedule. For example, a conventional controller of a NAND flash memory device may walk through all pages of the NAND flash memory device within a time period of one to two weeks. The present application recognizes that while this calibration frequency may provide satisfactory error rates for older flash technologies, newer non-volatile technologies, such as 3D TLC or QLC NAND flash, can require more frequent calibration, particularly when operating in elevated temperature environments. For example, in some cases, to maintain acceptable error rates, some non-volatile memory technologies may require read voltage thresholds to be calibrated on the order of hours or days rather than weeks.

As one specific example, consider the calibration of a 3D TLC NAND flash memory, where the read voltage threshold of each wordline is separately calibrated. Assuming 192 wordlines, seven read voltage thresholds (to define the eight possible cell states of TLC NAND), and three reads per read voltage threshold calibration, approximately 4,000 pages would have to be read for each calibration (e.g., 192×7×3≈4,000). Assuming that each read takes 100 microseconds, one calibration might take 0.4 second. If the 3D TLC NAND flash memory includes 100,000 blocks and each block is sequentially calibrated once per day, the calibration would take 40,000 seconds, which at about 11 hours would consume almost half the total internal bandwidth doing only calibrations. Obviously, for storage devices including a larger number of blocks, calibration of all blocks within the desired time frame would become impossible.

In view of foregoing, the present application recognizes that it would be useful and desirable to reduce the number of reads required to perform read voltage threshold calibrations while still performing calibration frequently enough to obtain acceptable bit error rates. The present application accordingly presents techniques that can be selectively applied to reduce the number of reads performed for read voltage threshold calibration.

In one exemplary embodiment, a non-volatile memory includes a plurality of cells each individually capable of storing multiple bits of data including bits of multiple physical pages including at least a first page and a second page. A controller of the non-volatile memory determines a first calibration interval for a first read voltage threshold defining a bit value in the first page and a different second calibration interval for a second read voltage threshold defining a bit value in the second page. The second calibration interval has a shorter duration than the first calibration interval. The controller calibrates the first and second read voltage thresholds for the plurality of memory cells in the non-volatile memory based on the determined first and second calibration intervals. In some embodiments, the controller can determine the second calibration interval based on relative probabilities of read failures of the lower page and upper page due to data retention effects.

In some embodiments, the number of reads performed in connection with read voltage threshold calibration can alternatively or additionally be reduced by calibrating read voltage thresholds for pages in a page group based on reading only a subset of pages in that page group.

In some embodiments, the number of reads performed in connection with read voltage threshold calibration can alternatively or additionally be reduced by deferring calibration of a read voltage threshold for at least a selected page of a page group when a bit error rate of the selected page does not satisfy a calibration threshold.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a high level block diagram of a data processing environment in accordance with one embodiment;

FIG. 1B is a more detailed block diagram of a flash card of the data storage system of FIG. 1A;

FIG. 2 depicts an exemplary NAND flash memory module in accordance with one embodiment;

FIG. 3 is a high level flow diagram of the flash management functions and data structures employed in flash management in accordance with one embodiment;

FIGS. 4A-4B depict initial and subsequent programmed threshold voltage distributions for an exemplary multi-level cell (MLC) NAND flash memory;

FIG. 5 is a graph illustrating read threshold voltages and bit values for an exemplary three-level cell (TLC) NAND flash memory;

FIG. 6 is a high level logical flowchart of an exemplary process for configuring calibration intervals for different pages based on the pages' relative probabilities of failure due to data retention effects;

FIG. 7 is a high level logical flowchart of an exemplary process for calibrating read voltage thresholds of a non-volatile memory in accordance with one embodiment;

FIG. 8 is a high level logical flowchart of an exemplary process for dynamically determining read voltage threshold values based on bit error rate feedback; and

FIG. 9A-9D depict the reductions in page reads made in connection with read voltage threshold calibration according to the various disclosed techniques.

DETAILED DESCRIPTION

With reference to the figures and with particular reference to FIG. 1A, there is illustrated a high level block diagram of an exemplary data processing environment 100 including a data storage system 120 having a non-volatile memory array as described further herein. As shown, data processing environment 100 includes one or more hosts, such as a processor system 102 having one or more processors 104 that process instructions and data. A processor system 102 may additionally include local storage 106 (e.g., dynamic random access memory (DRAM) or disks) that may store program code, operands and/or execution results of the processing performed by processor(s) 104. In various embodiments, a processor system 102 can be, for example, a mobile computing device (such as a smartphone or tablet), a laptop or desktop personal computer system, a server computer system (such as one of the POWER series of servers available from International Business Machines Corporation), or a mainframe computer system. A processor system 102 can also be an embedded processor system using various processors such as ARM, POWER, Intel x86, or any other processor combined with memory caches, memory controllers, local storage, I/O bus hubs, etc.

Each processor system 102 further includes an input/output (I/O) adapter 108 that is coupled directly (i.e., without any intervening device) or indirectly (i.e., through at least one intermediate device) to a data storage system 120 via an I/O channel 110. In some embodiments, data storage system 120 may be integral to a processor system 102. In various embodiments, I/O channel 110 may employ any one or a combination of known or future developed communication protocols, including, for example, Fibre Channel (FC), FC over Ethernet (FCoE), Internet Small Computer System Interface (iSCSI), InfiniBand, Transport Control Protocol/Internet Protocol (TCP/IP), Peripheral Component Interconnect Express (PCIe), etc. I/O requests communicated via I/O channel 110 include read requests by which a processor system 102 requests data from data storage system 120 and write requests by which a processor system 102 requests storage of data in data storage system 120.

Although not required, in the illustrated embodiment, data storage system 120 includes multiple interface cards 122 through which data storage system 120 receives and responds to I/O requests of hosts via I/O channels 110. Each interface card 122 is coupled to each of multiple Redundant Array of Inexpensive Disks (RAID) controllers 124 in order to facilitate fault tolerance and load balancing. Each of RAID controllers 124 is in turn coupled (e.g., by a PCIe bus) to non-volatile storage media, which in the illustrated example include multiple flash cards 126 bearing NAND flash memory. In other embodiments, alternative and/or additional non-volatile storage devices can be employed.

In the depicted embodiment, the operation of data storage system 120 is managed by redundant system management controllers (SMCs) 123, which are coupled to interface cards 122 and RAID controllers 124. In various embodiments, system management controller 123 can be implemented utilizing hardware or hardware executing firmware and/or software.

FIG. 1B depicts a more detailed block diagram of an exemplary embodiment of a flash card 126 of data storage system 120 of FIG. 1A. Flash card 126 includes a gateway 130 that serves as an interface between flash card 126 and RAID controllers 124. Gateway 130 is coupled to a general-purpose processor (GPP) 132, which can be configured (e.g., by program code) to perform pre-processing on requests received by gateway 130 and/or to schedule servicing of the requests by flash card 126. GPP 132 is coupled to a GPP memory 134 (e.g., Dynamic Random Access Memory (DRAM)) that can conveniently buffer data created, referenced and/or modified by GPP 132 in the course of its processing or data flowing through the gateway 130 destined for one or more of the flash controllers 140.

Gateway 130 is further coupled to multiple flash controllers 140, each of which controls a respective NAND flash memory system 150. Flash controllers 140 can be implemented, for example, by an Application Specific Integrated Circuit (ASIC) and/or a Field Programmable Gate Array (FPGA) and/or a microprocessor, and each have an associated flash controller memory 142 (e.g., DRAM). In embodiments in which flash controllers 140 are implemented with an FPGA, GPP 132 may program and configure flash controllers 140 during start-up of data storage system 120. After startup, in general operation flash controllers 140 receive read and write requests from gateway 130 that request to read data stored in NAND flash memory system 150 and/or to store data in NAND flash memory system 150. Flash controllers 140 service these requests, for example, by accessing NAND flash memory system 150 to read or write the requested data from or into NAND flash memory system 150 or by accessing a memory cache (not illustrated) associated with NAND flash memory system 150.

Flash controllers 140 implement a flash translation layer (FTL) that provides logical-to-physical address translation to enable access to specific memory locations within NAND flash memory systems 150. In general, a request received by flash controller 140 from a host device, such as a processor system 102, contains the logical block address (LBA) at which the data is to be accessed (read or written) and, if a write request, the write data to be stored to data storage system 120. The request may also specify the amount (or size) of the data to be accessed. Other information may also be communicated depending on the protocol and features supported by data storage system 120. The flash translation layer translates LBAs received from a RAID controller 124 into physical addresses assigned to corresponding physical location in NAND flash memory systems 150. Flash controllers 140 may perform address translation and/or store mappings between logical and physical addresses in a logical-to-physical translation data structure, such as a logical-to-physical translation table (LPT), which may conveniently be stored in flash controller memory 142.

NAND flash memory systems 150 may take many forms in various embodiments. In the embodiment shown in FIG. 1B, each NAND flash memory system 150 includes multiple (e.g., 32) individually addressable NAND flash memory storage devices 152. In the illustrated example, the flash memory storage devices 152 take the form of a board-mounted flash memory modules, for example, Multi-Level Cell (MLC), Three Level Cell (TLC), or Quad Level Cell (QLC) NAND flash memory modules. The effective storage capacity provided by flash memory storage devices 152 can be increased through the implementation of data compression, for example, by flash controllers 140 and/or high level controllers, such as GPPs 132, RAID controllers 124 or SMCs 123.

Referring now to FIG. 2, there is depicted a block diagram of an exemplary flash memory module 200 that can be utilized to implement any of the NAND flash memory storage devices 152 of FIG. 1B. Flash memory module 200 includes one or more memory die, each implementing at least one memory array 202 formed of a two-dimensional or three-dimensional array of NAND flash memory cells. As indicated in FIG. 2, the memory cells within memory array 202 are physically arranged in multiple blocks 204, each in turn including multiple physical pages 206. As discussed below, these pages can be grouped in page groups, which can each be formed, for example, of all the pages coupled to a common wordline, of all pages in one or more layers in a 3D NAND flash, of a set of pages in one or more layers, or generally of pages with similar characteristics.

As is known to those skilled in the art, NAND flash memory, such as that employed in memory array 202, must be erased prior to being programmed. Further, NAND flash memory can be (but is not required to be) constrained by its construction such that the smallest granule of storage that can be erased is a block 204 and the smallest granule of storage that can be accessed by a read or write request is fixed at the size of a single physical page 206. It should be appreciated in this regard that the LBAs provided by host devices correspond to logical pages within a logical address space, where each logical page typically has a size of 4 kilobytes (kB). Physical pages 206, in contrast, typically have a larger size, for example, 16 kB, and can thus host multiple logical pages.

Flash memory module 200 further includes a row decoder 210 through which word lines of memory array 202 can be addressed and a column decoder 212 through which bit lines of memory array 202 can be addressed. In addition, flash memory module 200 includes read/write circuitry 214 that enables the memory cells of a physical page 206 to be programmed or read in parallel. Flash controller 200 additionally includes control circuitry 205 that provides chip-level control of operation of memory array 202, including read and write accesses made to physical pages 206 in memory array 202, erasure of blocks 204, and the amplitude, duration and polarity of related voltages applied to memory array 202.

Having described the general physical structure of one exemplary embodiment of a data storage system 120, certain operational aspects of data storage system 120 are now described with reference to FIG. 3, which is a high level flow diagram of the flash management functions and data structures employed by a GPP 132 and/or flash controller 140 in accordance with one embodiment.

Data storage system 120 does not generally allow external devices (e.g., hosts) to directly address and/or access the physical memory locations within NAND flash memory systems 150. Instead, data storage system 120 is generally configured to present to host devices one or more logical volumes each having a contiguous logical address space, thus allowing host devices to read and write data to and from logical block addresses (LBAs) within the logical address space while permitting one or more of the various levels of controllers (e.g., system management controller 123, RAID controllers 124, flash controllers 140 and GPP 132) to control where the data that is associated with the various LBAs actually resides in the physical memory locations comprising NAND flash memory systems 150. In this manner, performance and longevity of NAND flash memory systems 150 can be intelligently managed and optimized. In the illustrated embodiment, each flash controller 140 performs logical-to-physical address translation for an associated set of LBAs using a logical-to-physical address translation data structure, such as logical-to-physical translation (LPT) table 300, which can be stored, for example, in the associated flash controller memory 142. It should be noted that the logical address supplied to flash controller(s) 140 may be different from the logical address originally supplied to data storage system 120, since various components within data storage system 120 may perform address translation operations between the external devices and the flash controller(s) 140.

Flash management code running on the GPP 132 tracks erased blocks of NAND flash memory system 150 that are ready to be used in ready-to-use (RTU) queues 306, which may be stored, for example, in GPP memory 134. In the depicted embodiment, flash management code running on the GPP 132 maintains one RTU queue 306 per channel or plane (i.e., per data bus), and an identifier of each erased block that is to be reused is enqueued in the RTU queue 306 corresponding to its channel or plane. A build block stripes function 320 performed by flash management code running on the GPP 132 constructs new block stripes for storing data and associated parity information from the erased blocks enqueued in RTU queues 306. The new block stripes are then queued to the flash controller 140 for data placement. Block stripes are preferably formed of blocks residing in different channels, meaning that build block stripes function 320 can conveniently construct a block stripe by drawing each block of the new block stripe from a different RTU queue 306. In general, build block stripes function 320 attempts to construct stripes from blocks of approximately equal health (i.e., expected remaining useful life).

In response to write request received from a host, such as a processor system 102, a data placement function 310 of flash controller 140 determines by reference to LPT table 300 whether the target LBA(s) indicated in the write request is/are currently mapped to physical memory page(s) in NAND flash memory system 150 and, if so, changes the status of each data page currently associated with a target LBA to indicate that the associated data is no longer valid. In addition, data placement function 310 allocates a page stripe if necessary to store the write data of the write request and any non-updated data (i.e., for write requests smaller than a logical page, the remaining valid data from a previous write to the same logical address which is not being overwritten and which must be handled in a read-modify-write manner) from an existing page stripe, if any, targeted by the write request, and/or stores the write data of the write request and any non-updated (i.e., still valid) data from an existing page stripe, if any, targeted by the write request to an already allocated page stripe which has free space left. The page stripe may be allocated from either a block stripe already allocated to hold data or from a new block stripe built by build block stripes function 320. In a preferred embodiment, the page stripe allocation can be based on the health of the blocks available for allocation and the “heat” (i.e., estimated or measured write access frequency) of the LBA of the write data. Data placement function 310 then writes the write data, associated metadata (e.g., cyclic redundancy code (CRC) and error correcting code (ECC) values), and parity information for the page stripe in the allocated page stripe. Flash controller 140 also updates LPT table 300 to associate the physical page(s) utilized to store the write data with the LBA(s) indicated by the host device. Thereafter, flash controller 140 can access the data to service host read requests by reference to LPT table 300 as further illustrated in FIG. 3.

Once all pages in a block stripe have been written, flash controller 140 places the block stripe into one of occupied block queues 302, which flash management code running on the GPP 132 utilizes to facilitate garbage collection. As noted above, through the write process, pages are invalidated, and therefore portions of the NAND flash memory system 150 become unused. The associated flash controller 140 (and/or GPP 132) eventually needs to reclaim this space through garbage collection performed by a garbage collector 312. Garbage collector 312 selects particular block stripes for garbage collection based on a number of factors including, for example, the health of the blocks 204 within the block stripes and how much of the data within the erase blocks 204 is invalid. In the illustrated example, garbage collection is performed on entire block stripes, and flash management code running on GPP 132 logs the block stripes ready to be recycled in a relocation queue 304, which can conveniently be implemented in the associated flash controller memory 142 or GPP memory 134.

The flash management functions performed by GPP 132 or flash controller 140 additionally include a relocation function 314 that relocates the data held in block stripes enqueued in relocation queue 304. To relocate such data, relocation function 314 issues relocation write requests to data placement function 310 to request that the valid data of the old block stripe be written to a new block stripe in NAND flash memory system 150. In addition, relocation function 314 updates LPT table 300 to remove the current association between the logical and physical addresses of the data. Once all remaining valid data has been moved from the old block stripe, the old block stripe is passed to dissolve block stripes function 316, which decomposes the old block stripe into its constituent blocks 204, thus disassociating the blocks 204. Each of the blocks 204 formerly forming the dissolved block stripe is then erased under the direction of flash controller 140 and/or the control circuitry 205 of the relevant flash memory module 200, and a corresponding program/erase (P/E) cycle count 334 for each erased block is incremented. Based on the health metrics of each erased block 204 (e.g., bit error rate (BER) metrics, uncorrectable errors, P/E cycle count, etc.), each erased block 204 is either retired (i.e., withdrawn from use) by a block retirement function 318 among the flash management functions executed on GPP 132, or alternatively, prepared for reuse by placing the block 204 on the appropriate ready-to-use (RTU) queue 306 in the associated GPP memory 134.

As further shown in FIG. 3, the flash management functions executed on GPP 132 and/or flash controller 140 additionally include a background health checker 330. Background health checker 330, which operates independently of the read and write requests of hosts such as processor systems 102, continuously determines one or more block health metrics 332 for blocks belonging to block stripes recorded in occupied block queues 302. Based on the one or more of the block health metrics 332, background health checker 330 places block stripes on relocation queue 304 for handling by relocation function 314. Key block health metrics 332 preferably monitored and recorded by background health checker relate to the bit error rate (BER) metrics observed for valid blocks and physical pages, and may include, for example, the worst page BER of each block, the mean page BER of each block, the rates of change of the worst page BER and mean page BER of each block, etc. In order to obtain the most accurate health estimate possible, health can be determined from an analysis of valid and invalid data, thereby ensuring that blocks containing almost entirely invalid data are fully analyzed to determine overall block health.

As described in greater detail below, one function of GPP 132 and/or flash controller 140 that can be incorporated within background health checker 330 or that can be separately implemented is the periodic adaptation (calibration) of read voltage thresholds 336 utilized to decode the data bits stored within the memory cells of memory arrays 202. These read voltage thresholds 336, which can be individually defined to any desired level of granularity (e.g., per-page, per page group within a block, per block, etc.), are preferably selected to improve one or more bit error metrics. As further depicted in FIG. 3, GPP 132 and/or flash controller 140 preferably track additional statistics to facilitate intelligent adaptation of read voltage thresholds 336. These additional statistics can include, for example, P/E cycle counts 334 identifying a number of program/erase cycles to which each block has been subjected and read counts 338 indicating a number of times a given memory unit (e.g., page, page group and/or block) has been read since being programmed. In addition, these statistics can include calibration triggers 340, such as per-memory unit counters indicating an elapsed amount of time since read voltage thresholds of that memory unit were adapted and/or per-block counters indicating a number of program/erase cycles that each block has been subjected to since the read voltage thresholds for that block were last adapted.

Referring now to FIGS. 4A-4B, there is depicted initial and subsequent programmed threshold voltage distributions (V_(TH)) for an exemplary multi-level cell (MLC) NAND flash memory. As shown in FIG. 4A, when a block 204 of a NAND flash memory storage device 152 is first put into service, each of the four voltage distributions 400 a, 400 b, 400 c and 400 d, respectively representing bit values 11, 10, 00 and 01, is tight and well defined. Consequently, the bit values of the various memory cells can be decoded with little or no error by application of initial read voltage thresholds VA 402 a, VB 402 b and VC 402 c.

Through use, the memory cells within block 204 will be damaged by, among other things, the voltage stress associated with program/erase (P/E) cycling. As a result of this damage, the distribution of memory cell voltages will no longer reflect the tight distributions shown in FIG. 4A, but will instead be characterized by broader and/or shifted voltage distributions 406 a, 406 b, 406 c, 406 d shown in FIG. 4B. Because of the degradation of the voltage distributions, bit values can be erroneously decoded if read voltage thresholds VA 402 a, VB 402 b and VC 402 c continue to be applied, and the bit error rate (BER) for reads to the block will consequently increase.

To reduce the BER for the block, GPP 132 and/or flash controller 140 periodically adapts (calibrates) the read voltage thresholds for one or more individual memory units (e.g., a page, page groups and/or a block) either negatively (not illustrated) or positively (as shown explicitly in FIG. 4B). For example, in FIG. 4B read voltage thresholds VA 402 a, VB 402 b and VC 402 c are each shifted positively by an individually determined offset ΔA, ΔB or ΔC selected to reduce and/or minimize the BER. Thus, in the example of FIG. 4B, read voltage thresholds VA 402 a, VB 402 b and VC 402 c are replaced by read voltage thresholds VA+ΔA 404 a, VB+ΔB 404 b and VC+ΔC 404 c, respectively. The signs and magnitudes of the read voltage threshold offsets selected by GPP 132 and/or flash controller 140 can and will vary over the lifetime of the block based on various environmental factors (e.g., temperature) and use factors (e.g., read disturbs, write disturbs, erase disturbs, data retention effects, etc.).

Although FIGS. 4A-4B illustrate the calibration of the three read voltage thresholds utilized to define the four possible bit value combinations represented by the charge stored on the floating gate of an MLC flash memory cell, it will be appreciated that the same principles are applicable to other types of non-volatile memories, including TLC and QLC NAND flash. For example, FIG. 5 is a graph illustrating the seven read threshold voltages defining the eight possible bit value combinations that can be represented by the charge stored on the floating gate of an exemplary TLC NAND flash memory cell. Like the read voltage thresholds employed in MLC NAND flash, the seven read voltage thresholds of the TLC NAND flash are subject to repeated calibration and positive or negative adaptation during use.

In the art, the least significant bit value of a TLC NAND flash cell is said to form a portion of a “lower page,” the next least significant bit forms a portion of the “upper page,” and the most significant bit forms a portion of the “extra page.” For QLC, this tripartite division of page types is augmented by a fourth bit of data within an “additional page.” FIG. 5 illustrates the various read voltage thresholds utilized to distinguish the different possible bit values of the various pages in one embodiment of TLC NAND flash memory. For example, a single read voltage threshold Lp0 is utilized to distinguish between the two possible least significant bit (i.e., lower page) values, with a voltage less than Lp0 representing a 0 and a voltage greater than Lp0 representing a 1. Two read voltage thresholds, Up0 and Up1, are further utilized to distinguish between the two possible middle bit (i.e., upper page) values, with a voltage between Up0 and Up1 representing a 1 and a voltage less than Up0 or greater than Up1 representing a 0. Finally, four read voltage thresholds Xp0-Xp3 are utilized to distinguish between the two possible most significant bit (i.e., extra page) values, with a 0 being represented by a voltage less than Xp0 or between Xp1 and Xp2 or greater than Xp3 and a 1 being represented by a voltage between Xp0 and Xp1 or between Xp2 and Xp3. Of course, in other embodiments, different assignments between bit values and read voltage thresholds are possible.

The present application appreciates that the probability of a read error in a page due to data retention effects has a direct relationship to the number of read voltage thresholds utilized to distinguish between the bit values on that page. Thus, if there are N cells per page and the probabilities of an error occurring in a single cell of an additional, extra, upper, and lower page due to data retention effects are given by p_(a), p_(x), p_(u), and p_(l), respectively, then it necessarily follows that Np_(a)>Np_(x)>Np_(u)>Np_(l). Consequently, in accordance with at least one embodiment, instead of implementing a single calibration interval at which all blocks are recalibrated, multiple different calibration intervals are employed so that pages having a greater probability of failing due to data retention effects are recalibrated more frequently than those that are less likely to fail due to data retention effects. As a result, fewer calibration reads need be performed overall, reducing the total calibration overhead and the associated consumption of power and throughput.

Referring now to FIG. 6, there is depicted a high level logical flowchart of an exemplary process for configuring calibration intervals for different pages based on the pages' relative probabilities of failure due to data retention effects. The process of FIG. 6 can be performed by a controller, such as a GPP 132 and/or flash controller 140. For example, in one embodiment, which will hereafter be assumed, the process is performed by the GPP 132 of each flash card 126 through execution of program instructions stored within its associated GPP memory 134.

The process of FIG. 6 begins at block 600 and then proceeds to block 602, which illustrates GPP 132 configuring a calibration interval for lower pages. In at least some preferred embodiments, the calibration interval C₁ for lower pages is initially configured based on device characterization data, which indicate a data retention period after which tunneling effects will cause the charge on the floating gates of the cells to decrease to the point that an unacceptably high bit error rate (e.g., a bit error rate sufficient to cause a page read failure or a percentage thereof) will be observed. In subsequent reconfigurations, if any, the lower page calibration interval can additionally be configured based on environmental data (e.g., temperature, humidity, etc.) and/or operational data, such as block health metrics 332 and/or P/E cycle counts 334. It should be appreciated that the lower page calibration interval C₁ for lower pages can be expressed in either chronological time (e.g., second, minutes, hours, and/or days) and/or wear (e.g., P/E cycles).

At block 604, GPP 132 additionally configures calibration intervals of upper pages, extra pages, and/or additional pages (depending on the number of page types supported by the implemented memory devices) based on the lower page calibration interval and relative probability of failure due to data retention effects. For example, in one exemplary implementation, GPP 132 configures the calibration interval of the upper pages, extra pages, and/or additional pages at a rate directly proportional to the various page types' relative probabilities of failure. If the probabilities of read failure of additional pages, extra pages, upper pages, and lower pages are represented by p_(a), p_(x), p_(u), and p_(l), respectively, then frequency of calibrations of the additional pages C_(a) can be given by C_(l)×p_(a)/p_(l) (since additional pages have higher probability of read failures, they have to be calibrated more frequently). Equivalently, the frequency can be inverted to calculate the time interval between calibrations, in which case the time interval between calibrations of the additional pages could be given by C_(a)=C_(l)×p_(l)/p_(a). Similarly for other page types, the calibration interval C_(x) of the extra pages could be given by C_(x)=C_(l)×p_(l)/p_(x), and the calibration interval C_(u) of the upper pages could be given by C_(u)=C_(l)×p_(l)/p_(u). As noted above, in some embodiments, the probabilities are further directly related to the number of read voltage thresholds per page type, meaning that in these embodiments the additional page calibration interval C_(a) would have a duration one-eighth that of lower page calibration interval C_(l), extra page calibration interval C_(x) would have a duration one-fourth that of lower page calibration interval C₁, and upper page calibration interval C_(u) would have a duration one-half that of lower page calibration interval C₁.

Following blocks 602 and 604, GPP 132 determines at block 606 whether or not the calibration intervals of the various page types should be reconfigured. For example, GPP 132 may determine to reconfigure the calibration intervals based on the elapse of time, an average number of P/E cycles across all of its blocks, and/or environmental data, such as temperature. If GPP 132 determines at block 606 not to reconfigure the calibration intervals, the process iterates at block 606. If, however, GPP 132 determines to reconfigure the calibration intervals at block 606, the process of FIG. 6 returns to blocks 602-604, which have been described.

With reference now to FIG. 7, there is depicted a high level logical flowchart of an exemplary process for adapting (calibrating) read voltage thresholds of a non-volatile memory in accordance with one embodiment. The exemplary process, which is preferably performed by a controller, such as a GPP 132 and/or flash controller 140, can be incorporated within background health checker 330 or can be separately implemented. In various embodiments, the read voltage thresholds can be individually defined for a memory unit at any desired level of granularity (e.g., per-page, per page group within a block, per block, etc.). For ease of understanding, the following description assumes that each page group within a block is formed of pages that are of the same page type (e.g., lower pages, upper pages, extra pages, or additional pages) and that share a common set of read voltage threshold(s) 336.

The process of FIG. 7 begins at block 700 and then proceeds to block 702, which illustrates the controller selecting a next block 204 of memory for read voltage threshold calibration. The controller additionally selects a next page group from the selected block at block 704. At block 706, the controller determines whether or not the calibration interval for the page type of the pages forming the currently selected page group has elapsed. The controller may make the determination depicted at block 706, for example, by reference to per-page group counters within calibration triggers 340 indicating an elapsed amount of time since read voltage thresholds of that page group were most recently adapted and/or per-page group counters within calibration triggers 340 indicating a number of P/E cycles that the block has been subjected to since the read voltage thresholds for that page group were last adapted.

In response to a negative determination at block 706, read threshold calibration is not performed for the selected page group, and the process returns to block 704. If, however, the controller determines at block 706 that the calibration interval for the page type of the pages forming the currently selected page group has elapsed, the process proceeds in some embodiments directly to block 710 (while omitting optional block 708) and, in other embodiments, proceeds to block 710 through optional block 708.

Block 708 shows that, in some embodiments, the controller does not perform a calibration read of each page in the selected page group, but instead decreases the number of calibration reads performed to calibrate the read voltage threshold(s) of the selected page group by performing calibration read(s) of, at most, only a smaller representative subset of sample page(s). Accordingly, in embodiments including optional block 708, controller selects zero or more sample pages in the selected page group that will be read to calibrate the read voltage threshold(s) for all pages in the selected page group. When fewer sample pages are selected than pages are present in a page group, then the optimal threshold voltages found for these selected one or more pages are used to determine the read voltage thresholds for the entire page group. The determined read voltage thresholds for the page group will later be applied on read operations for any page in the page group even though the page was not used in the calibration process. This sampling comes at the cost of a potential increase in the BER for pages that have not been read during the calibration process. This potential increase in the BER is acceptable as the BER will be below the calibration threshold.

In some embodiments, the controller determines how many and/or which sample page(s) to select at block 708 utilizing a random or pseudo-random selection. In other embodiments, the controller may make an informed selection of how many and/or which sample page(s) to represent the page group, for example, based on characterization data and/or operational data. For example, in one embodiment, the controller may determine how many and which pages will be read from the selected page group based on bit error rates (BERs) recorded as part of block health metrics 332. In some cases, the controller may determine based on operational statistics and/or characterization data that certain page groups have low BERs and may therefore select zero sample pages at block 708. In this extreme case, calibration of the read threshold voltage(s) of the selected page group is deferred until a subsequent iteration of the process shown in FIG. 7.

Following block 708, if implemented, the controller adapts the read voltage threshold(s) of the page group as indicated by the calibration reads, as shown at block 710. One embodiment of this adaptation process is described further below with reference to FIG. 8. Following block 710, the controller determines at block 712 whether or not all page groups of the selected block have been processed. If not, the process of FIG. 7 returns to block 704 and selects the next page group for calibration, which has been described earlier. If, however, controller determines at block 712 that all page groups in the currently selected block have been processed, the process returns to block 702, where the controller selects a next block of non-volatile memory for read voltage threshold calibration.

In some embodiments of the process of FIG. 7, the range and number of candidate offset values employed for page reads made during the course of calibration at block 710 can be static and can be determined, for example, based on characterization data for the memory. In alternative embodiments such as that disclosed in FIG. 8, the range and/or number of candidate offset values represented in an offset set can instead be determined by the controller dynamically based on bit error rate (BER) feedback.

With reference now to FIG. 8, there is illustrated a high level logical flowchart of an exemplary process for dynamically adapting read voltage threshold values for a page based on bit error rate feedback. The process illustrated in FIG. 8 can be utilized to implement, for example, the process shown at block 710 of FIG. 7.

The process of FIG. 8 begins at block 800 and then proceeds to block 802, which illustrates the controller selecting a first or next page of the page group for processing. If optional block 708 of FIG. 7 is implemented, the page selected at block 802 is selected from among only the sample pages identified at block 708. In alternative embodiments in which block 708 is omitted, the controller selects from among all pages in the page group at block 802. At block 804, the controller reads the selected page of the selected page group utilizing the current read voltage threshold(s) for that page, which corresponds to use of a read voltage offset set of {0}.

Following block 804, the process of FIG. 8 proceeds in some embodiments to optional block 806, which illustrates the controller determining if the BER obtained by reading the selected page satisfies (e.g., exceeds) a calibration threshold (e.g., a BER corresponding to a page read failure or a percentage thereof). If not, controller may determine that no further reads of the selected page are required for calibration, and the process passes to block 816, which is described below. If, on the other hand, the controller determines at block 806 that the BER obtained by reading the selected page with a current read voltage threshold(s) satisfies the calibration threshold, or if optional block 806 is omitted, the process passes to block 807.

Block 807 depicts the controller twice reading the current page utilizing read voltage threshold(s) shifted positively and negatively by an initial offset set {±Δ}, where A represents an increment of voltage change. The controller then determines at block 808 whether or not the read voltage threshold offset resulting in the optimal BER for the page was found utilizing the initial offset set. For example, the controller can determine that the optimal BER has been found if reading the page utilizing the offsets −Δ and +Δ results in greater or equal BERs than utilizing an initial offset of 0. In another embodiment, the controller can determine that the optimal BER has been found if reading the page utilizing the offsets −Δ and +Δ would not reduce the BER more than some configured tolerance BER, which can be given, for example, as an absolute number (certain number of bit errors are tolerated for the given page), or as a percentage of the BER. There can also be additional limitations when the tolerance BER can be applied, for example only in early life, or under low BER values. In response to a determination at block 808 that the offset providing the optimal BER has been successfully identified, the process proceeds from block 808 to block 814, which is described below. However, in response to a determination at block 808 that reading data from the memory unit utilizing the current offset set did not definitively identify the offset providing the optimal BER for the page, the process proceeds from block 808 to block 810.

Block 810 illustrates the controller creating a new offset set to test by utilizing the offset in the previous offset set that provided the lowest BER as the end value of new offset range and by selecting a number of additional offsets and/or offset multipliers. In some embodiments, the range and/or number of offsets selected for inclusion in the new offset set at block 810 can be determined based on the BER slope observed for the different values in the previous offset set. For example, in some embodiments, at block 810 the controller employs three offsets in the offset set, but varies a multiplier by which the base offset (Δ) is multiplied based on the BER slope. In other embodiments, both the number of offsets in the new set of offsets and the offset multiplier are varied. In general, the controller may utilize more offsets and/or use larger offset multiplier(s) if the BER slope is relatively large and utilize fewer offsets and/or smaller offset multiplier(s) if the BER slope is relatively small. As one example, assuming that testing utilizing the initial offset set revealed that the offset of −Δ provided the lowest BER, at block 810 the controller may create a new offset set {-Δ, −2Δ, −3Δ}. In some embodiments, the offset multiplier is varied based on the number of offset sets evaluated until that point at block 810. For example, offset multiplier(s) may be larger for the first offset set generated at block 810, and the offset multiplier(s) may get reduced for each offset set generated at block 810. Such embodiments have the advantage of the improved confidence that the calibration process would terminate successfully in a certain number of steps, even for blocks or technologies that have unstable/unpredictable BER slopes.

At block 812, the controller reads data from the memory unit utilizing the read voltage threshold offsets in the updated offset set and notes the BER obtained through the use of each offset. The controller then determines at block 808 whether or not the offset resulting in the optimal BER for the memory unit was found utilizing the updated offset set. If not, the process returns to block 810 and following blocks, which have been described. In so doing, the controller incrementally approaches the optimal or near optimal offset that results in the optimal BER for the page. If the controller instead determines at block 808 that it has found the offset that results in the optimal BER for the page, the controller records the read voltage threshold(s) providing the optimal BER for the page (block 814).

The process passes from block 814 to block 816, which illustrates the controller determining if all relevant pages of the page group have been processed. If not, the process returns to block 802, which has been described. If, however, all relevant pages of the page group have been processed, the controller determines and records in read voltage thresholds 336 the new read voltage thresholds to be employed to read from page in the page group during the next calibration interval (block 818). In cases in which a calibration read was performed to only a single sample page, the read voltage threshold(s) recorded at block 814 may be stored in read voltage thresholds 336 at block 818. In cases in which multiple pages were subject to calibration read(s), the controller may determine the per-page group read voltage threshold(s) by taking an average or weighted average of the read voltage thresholds recorded at block 814 for the individual pages. Following block 818, the process of FIG. 8 ends at block 820.

In some embodiments, it may be useful and desirable to track the read voltage threshold 336 for each page group using a combination of multiple parameters. For example, in one embodiment, read voltage thresholds 336 may be specified by applying an offset to the default read voltage threshold, where the offset comprises a “base” component (V_(base)) that tracks a permanent deviation from the default read voltage threshold (e.g., due to P/E cycling) and a “temporary” component (V_(temp)) that tracks temporary changes of read voltage thresholds (e.g., due to data retention or read disturb effects).

With reference now to FIGS. 9A-9D, the reduction in page reads made in connection with read voltage threshold calibration according to the various disclosed techniques is illustrated. As indicated above, a controller may employ a single one of the disclosed techniques to reduce the number of calibration reads or may employ multiple of the disclosed techniques in combination. Further, the controller may dynamically adapt which of the disclosed technique(s) is employed, for example, characterization data, block health metrics 332, P/E cycle counts, a number of page read failures, etc.

Each of FIGS. 9A-9D illustrates an exemplary block 900 of non-volatile memory comprising a plurality of page groups 902 numbered 1, 2, 3, 4, 5, 6, 7, 8, . . . . Each of these page groups, in turn, includes a multiplicity of physical pages of storage.

FIG. 9A illustrates the calibration reads performed in connection with read voltage threshold calibration of block 900 in an embodiment omitting optional blocks 708 and 806. Specifically, FIG. 9A shows that when the calibration interval for a page group 902 elapses, the page group 902 is calibrated utilizing three or more (denoted as 3+) calibration reads to each page 904 in the page group 902. It should be noted that in this embodiment, the overall number of page reads performed in connection with read voltage threshold calibration are still reduced as compared to prior art systems because page groups 902 formed of upper pages, extra pages, and additional pages are calibrated less frequently than page groups formed of lower pages

FIG. 9B depicts the reductions of calibration reads performed in connection with read voltage threshold calibration of block 900 in an embodiment including optional block 708, but omitting optional block 806. In this example, based on the decision made in block 706 the controller selects zero sample pages to read from (i.e., does not calibrate) page groups that execution statistics and/or characterization data indicate are unlikely to need calibration. At block 708, the controller selects two sample pages from page groups that are indicated at block 706 as suitable for calibration by execution statistics and/or page type and/or characterization data indicating a higher variability among page BERs. At block 708, the controller also selects a single sample page to read from each other page group. Utilizing these settings, during the course of a sequence of read voltage threshold calibration, the controller will perform no calibration read to pages within page groups 1 and 8, will perform three or more calibration reads to a single sample page in each of page groups 2-3 and 5-6, and will perform three or more calibration reads to two sample pages in each of page groups 4 and 7.

FIG. 9C illustrates the calibration reads performed in connection with read voltage threshold calibration of block 900 in an embodiment omitting optional block 708 and including optional block 806. For simplicity, this example shows an embodiment in which the controller selects all page groups for calibration at block 706, so all reductions in calibration reads are made by block 806 only. In this example, the controller tests each page 904 with a single read utilizing the current read voltage threshold(s) prior to proceeding to additional calibration reads. As a result, during the course of a sequence of read voltage threshold calibration, the controller may perform only a single calibration read to each page 904 in page groups 1-3, 5-6, and 8 and perform three or more calibration reads to each page in page groups 4 and 7.

Finally, FIG. 9D depicts the calibration reads performed in connection with read voltage threshold calibration of block 900 in an embodiment including both optional blocks 708 and 806. In this example, the controller greatly reduces the number of calibration reads by employing execution statistics and/or characterization data at block 706, by implementing page sampling at block 708, and by testing the bit error rate obtained from calibration reads against a calibration threshold at block 806. As shown, during the course of a sequence of read voltage threshold calibration, the controller may perform no calibration read to pages within page groups 1 and 8, perform a single calibration read to a single page in each of page groups 2-3 and 5-6, and perform three or more calibration reads to one page in each of page groups 4 and 7.

As has been described, in at least one embodiment, a non-volatile memory includes a plurality of cells each individually capable of storing multiple bits of data including bits of multiple physical pages including at least a first page and a second page. A controller of the non-volatile memory determines a first calibration interval for a first read voltage threshold defining a bit value in the first page and a different second calibration interval for a second read voltage threshold defining a bit value in the second page. The second calibration interval has a shorter duration than the first calibration interval. The controller calibrates the first and second read voltage thresholds for the plurality of memory cells in the non-volatile memory based on the determined first and second calibration intervals.

The present invention may be a system, a method, and/or a computer program product. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention.

The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire.

Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device.

Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++ or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions.

These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks.

The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.

While the present invention has been particularly shown as described with reference to one or more preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. For example, although aspects have been described with respect to a data storage system including a flash controller that directs certain functions, it should be understood that present invention may alternatively be implemented as a program product including a storage device storing program code that can be processed by a processor to perform such functions or cause such functions to be performed. As employed herein, a “storage device” is specifically defined to include only statutory articles of manufacture and to exclude signal media per se, transitory propagating signals per se, and energy per se.

In addition, although embodiments have been described that include use of a NAND flash memory, it should be appreciated that embodiments of the present invention can also be used with any other type of non-volatile random access memory (NVRAM).

The figures described above and the written description of specific structures and functions below are not presented to limit the scope of what Applicants have invented or the scope of the appended claims. Rather, the figures and written description are provided to teach any person skilled in the art to make and use the inventions for which patent protection is sought. Those skilled in the art will appreciate that not all features of a commercial embodiment of the inventions are described or shown for the sake of clarity and understanding. Persons of skill in this art will also appreciate that the development of an actual commercial embodiment incorporating aspects of the present inventions will require numerous implementation-specific decisions to achieve the developer's ultimate goal for the commercial embodiment. Such implementation-specific decisions may include, and likely are not limited to, compliance with system-related, business-related, government-related and other constraints, which may vary by specific implementation, location and from time to time. While a developer's efforts might be complex and time-consuming in an absolute sense, such efforts would be, nevertheless, a routine undertaking for those of skill in this art having benefit of this disclosure. It must be understood that the inventions disclosed and taught herein are susceptible to numerous and various modifications and alternative forms. Lastly, the use of a singular term, such as, but not limited to, “a” is not intended as limiting of the number of items. 

What is claimed is:
 1. A method of adapting read voltage thresholds in a non-volatile memory including a plurality of cells each individually capable of storing multiple bits of data including bits of multiple physical pages including at least a first page and a second page, the method comprising: a controller determining a first calibration interval for a first read voltage threshold defining a bit value in the first page; the controller determining a different second calibration interval for a second read voltage threshold defining a bit value in the second page, wherein the second calibration interval has a shorter duration than the first calibration interval; and calibrating the first and second read voltage thresholds for the plurality of memory cells in the non-volatile memory based on the determined first and second calibration intervals.
 2. The method of claim 1, wherein the first page is a lower page and the second page is an upper page.
 3. The method of claim 1, wherein the first page is an upper page and the second page is an extra page.
 4. The method of claim 1, and further comprising the controller determining the second calibration interval based on differing probabilities of read failures of the first and second pages due to data retention effects.
 5. The method of claim 1, wherein: the non-volatile memory includes a block including a plurality of pages including the first and second pages; the block comprises a plurality of page groups formed from the plurality of pages, wherein each of plurality of page groups includes multiple pages; and the method further comprises the controller, when calibrating read voltage thresholds for the block, calibrates read voltage thresholds for at least one page group among the plurality of page groups based on reading only a subset of pages in that page group.
 6. The method of claim 1, and further comprising: deferring a scheduled calibration of a selected page among the first and second pages based on a bit error rate obtained by reading the selected page.
 7. The method of claim 6, wherein: the non-volatile memory includes a block including a plurality of pages including the first and second pages; the block comprises a plurality of page groups formed from the plurality of pages; and the deferring includes deferring calibration of all pages in a page group containing the selected page.
 8. A data storage system, comprising: a controller for a non-volatile memory including a plurality of cells each individually capable of storing multiple bits of data including bits of multiple physical pages including at least a first page and a second page, wherein the controller is configured to perform: a controller determining a first calibration interval for a first read voltage threshold defining a bit value in the first page; the controller determining a different second calibration interval for a second read voltage threshold defining a bit value in the second page, wherein the second calibration interval has a shorter duration than the first calibration interval; and calibrating of the first and second read voltage thresholds for the plurality of memory cells in the non-volatile memory based on the determined first and second calibration intervals.
 9. The data storage system of claim 8, wherein the first page is a lower page and the second page is an upper page.
 10. The data storage system of claim 8, wherein the first page is an upper page and the second page is an extra page.
 11. The data storage system of claim 8, wherein the controller is further configured to perform: the controller determining the second calibration interval based on differing probabilities of read failures of the first and second pages due to data retention effects.
 12. The data storage system of claim 8, wherein: the non-volatile memory includes a block including a plurality of pages including the first and second pages; the block comprises a plurality of page groups formed from the plurality of pages, wherein each of plurality of page groups includes multiple pages; and the controller, when calibrating read voltage thresholds for the block, calibrates read voltage thresholds for at least one page group among the plurality of page groups based on reading only a subset of pages in that page group.
 13. The data storage system of claim 8, wherein the controller is configured to perform: deferring a scheduled calibration of a selected page among the first and second pages based on a bit error rate obtained by reading the selected page.
 14. The data storage system of claim 13, wherein: the non-volatile memory includes a block including a plurality of pages including the first and second pages; the block comprises a plurality of page groups formed from the plurality of pages; and the deferring includes deferring calibration of all pages in a page group containing the selected page.
 15. A computer program product, the computer program product comprising a computer readable storage medium having program instructions embodied therewith, the program instructions being executable by a controller of a non-volatile memory including a plurality of cells each individually capable of storing multiple bits of data including bits of multiple physical pages including at least a first page and a second page, wherein, when executed, the program instructions cause the controller to perform: determining a first calibration interval for a first read voltage threshold defining a bit value in a first page of the non-volatile memory; determining a different second calibration interval for a second read voltage threshold defining a bit value in a second page, wherein the second calibration interval has a shorter duration than the first calibration interval; and calibrating the first and second read voltage thresholds for the plurality of memory cells in the non-volatile memory based on the determined first and second calibration intervals.
 16. The program product of claim 8, wherein the first page is a lower page and the second page is an upper page.
 17. The program product of claim 8, wherein the first page is an upper page and the second page is an extra page.
 18. The program product of claim 8, wherein the controller is further configured to perform: the controller determining the second calibration interval based on differing probabilities of read failures of the first and second pages due to data retention effects.
 19. The program product of claim 8, wherein: the non-volatile memory includes a block including a plurality of pages including the first and second pages; the block comprises a plurality of page groups formed from the plurality of pages, wherein each of plurality of page groups includes multiple pages; and the controller, when calibrating read voltage thresholds for the block, calibrates read voltage thresholds for at least one page group among the plurality of page groups based on reading only a subset of pages in that page group.
 20. The program product of claim 8, wherein the controller is configured to perform: deferring a scheduled calibration of a selected page among the first and second pages based on a bit error rate obtained by reading the selected page. 